In present-day automotive finishing, various substrates, such as, for example, bodies and bodywork parts made of metal or plastic, are painted. In addition to single-coat finishes, multicoat paint systems are frequently also built up. Multicoat paint systems on metallic substrates consist in general of an electrocoat, a primer-surfacer coat, a basecoat and a clearcoat. In the case of plastics substrates, either single-coat finishes or, again, multicoat paint systems are built up. In the latter case, the customary primer-surfacer, single-coat topcoat, basecoat and clearcoat coating compositions that are employable in the finishing of plastics are used, their selection and use being known to the skilled person.
The exacting technological and optical requirements imposed on paint finishes in the sector of the present-day automobile industry, and also the functions and technological properties of each of the individual paint coats identified above, are known to the skilled person. The clearcoats determine such essential optical properties as, for example, the gloss and the distinctiveness of image (DOI) and/or the appearance of the painted surface. This appearance is influenced to a particular degree by the roughness or waviness of the surface. Whereas a rough surface leads to a relatively nonuniform, optically unadvantageous aspect, a particularly smooth, regular surface possesses a good appearance. The clearcoat coating composition with which the clearcoat is produced must therefore have corresponding properties so that the appearance of the resultant finish is in keeping with the exacting requirements of the automobile industry. These requirements include, more particularly, good leveling properties and good filling capacity on the part of the clearcoat coating composition. By this means it is possible, for example, to compensate unevennesses in the substrate, in order to obtain a surface of maximum smoothness. In conventional multicoat paint systems, this is commonly brought about by the coats situated below the clearcoat, more particularly by the primer-surfacer coat. In order to boost the efficiency of painting operations, at the same time as aiming for cost minimization, however, it is becoming more and more important for the clearcoat coating compositions as well to have the stated properties. Thus, for example, as part of painting operations, attempts are made to be able to use steel substrates as well, which have a greater roughness and are therefore more cost-effective. This higher roughness, however, has to date often not been adequately masked by the coats situated beneath the clearcoat, with the consequence that the clearcoat coating composition used must also have a good filling capacity, in order to achieve a good appearance. A further factor is that, as part of operational optimization, the flashing and drying times of the non-topcoat coats are often reduced, with the consequence that a maximum flow effect is not achieved here. Furthermore, attempts are being made to reduce the thicknesses of the coats to an ever further extent, or even to omit individual coats entirely, as part of innovative coating operations. For these reasons as well, the function of hiding unevennesses is increasingly being shifted to the clearcoat as well. It is important, correspondingly, that clearcoat coating compositions have a good filling capacity and hence contribute to an outstanding appearance on the part of the painted surface.
EP 0 837 891 B1 discloses a method for the multicoat painting of automobiles using transparent coating materials which are distinguished, for example, by a good topcoat appearance, more particularly gloss. The coating material here comprises a specific binder combination, using, among others, a (meth)acrylate copolymer, a polyester, a blocked isocyanate and a melamine resin.
Within the automobile industry, however, there continues to be a need for improvements with regard to the visual appearance of painted surfaces.
A further factor is that nowadays, as another requirement imposed on coating materials, more particularly on solventborne clearcoat coating compositions, increasing the nonvolatile fraction (NVF) of these compositions stands as the focal point of the coatings industry. Through a slight increase in the NVF and, consequently, a reduction in the fraction of volatile organic compounds, more particularly solvents, the systems can be made more eco-friendly. Moreover, just a small increase in the NVF produces a lower level of materials consumption/a higher productivity in coating, and hence, as part of large-scale industrial coating plants with correspondingly high throughput rates, achieves a large saving on material and ultimately operates significantly more economically and in turn in a more eco-friendly manner. It is an important objective on the part of the coatings industry, accordingly, to achieve a steady increase in the NVF or reduction in VOC emissions while at the same time preserving the technological properties of the respective coating compositions—clearcoat coating compositions for example.
In spite of efforts in this vein by the automobile industry, it has not to date been possible satisfactorily to achieve the above-identified properties for clearcoat coating compositions. Accordingly, a clearcoat coating composition which has a good filler capacity and good leveling and can therefore be utilized for producing coatings on substrates, more particularly bodies and bodywork parts, made of metal or plastic, from the automotive finishing segment is needed. The coatings ought accordingly to have a high gloss and more particularly an outstanding appearance. Additionally, it is desired toincrease the nonvolatile fraction of the clearcoat coating composition while at the same time preserving the technological properties, more particularly the appearance, and, hence, to acheive an economic and environmental profile which is improved by comparison with technologically comparable or even poorer compositions.